This invention relates to apparatus for monitoring earth faults on an exciter winding on the rotor of a generator and is concerned more particularly with monitoring faults on rotor windings of large turbogenerators such as are employed by utilities in large power stations.
The d.c. supply to the rotor winding of large turbogenerators is derived (in the majority of cases) from an a.c. machine whose shaft is mechanically coupled to the rotor at the slip ring end, the 3-phase output being rectified by a static full-wave diode bridge. This is fully insulated from earth or ground and thus normally the combination of a.c. machine/rectifier bridge/rotor winding (hereafter referred to as the excitation system) floats relative to earth with an insulation resistance of more than 10 M.OMEGA..
Occasionally the insulation resistance to earth (rotor body) falls. This can be caused by a variety of mechanisms, for example the build up of brush dust around the slip rings, the presence of steam in the area of the rectifier cubicle or potentially more serious the degradation of the rotor insulation itself. Since the excitation system floats, the existence of a single insulation defect is not in itself sufficient to cause damage, as there is no return path for current flowing to earth. However if a second defect occurs current can flow between the defects via the rotor body (say) shunting a section of the rotor winding. If a significant proportion of the rotor current is diverted through the rotor body, the potential damage that can be caused is very high because of localised heating of the defect sites. This may take the form of an intense arc between the winding and rotor body, and molten copper and steel will be thrown out from the rotor by centrifugal action.
Most large turbogenerators nowadays are fitted with rotor "earth leakage" alarm equipment. Such equipment makes use of a low voltage d.c. supply which is used to bias the whole excitation system with respect to earth. If the insulation resistance at any point in the winding falls, there will be an increase in the leakage current flowing in the bias circuit. A replay is provided which operates at a predetermined level of this leakage current to give an alarm signal. The absolute magnitude of the current in such a circuit will depend not only on the resistance of the defect or defects and on the exciter voltage but also on the position of the defect or defects in the winding. The circuit would normally be arranged so that, on receipt of the first alarm, the insulation resistance is still sufficiently high that the rotor winding is not yet at significant risk. It will be appreciated that it may be several years from the first indication of low insulation resistance until the final failure of the rotor winding. Thus, when such an alarm is received, tests are made to check the validity of the alarm. If the fault is confirmed and economic reasons dictate continued operation of the equipment, then the problem arises of a second defect occurring as discussed above. The present invention is directed to this problem. It is envisaged that, if a conventional "earth leakage" alarm circuit gives an alarm signal and the equipment is to continue in use, then the apparatus of the present invention will be fitted.